The present invention relates to an electrolytic cell and process for producing hydrogen peroxide at a high current efficiency.
There is concern about adverse influences of pollution by industrial and household wastes, such as air pollution and the deterioration of water quality in rivers and lakes, on the environment and the human body, and there is an urgent need to take technical measures to eliminate those problems. For example, a chemical such as chlorine has been used in the treatment of drinking water, sewage, and wastewater for the purpose of decoloring, COD reduction, and sterilization. However, since large chlorine doses result in the generation of hazardous substances, e.g., environmental hormones (exogenous endocrine disruptors) and carcinogenic substances, the addition of chlorine tends to be prohibited.
The incineration of wastes can generate carcinogenic substances (dioxins) in the emission gas depending on combustion conditions and thereby adversely affect the ecosystem. The safety of waste incineration is hence regarded as questionable. A novel method of water treatment with hydrogen peroxide has been proposed for eliminating the problem concerning water treatments.
Hydrogen peroxide is a chemical suitable for sterilization in such water treatments and the like. Besides being suitable for water treatments, hydrogen peroxide is useful as a basic chemical indispensable to the food, medicine, pulp, textile, and semiconductor industries. Future uses thereof which are attracting particular attention include the cleaning of electronic parts and the sterilization of medical instruments and apparatus.
In power plants and factories where seawater is used, a technique for preventing the attachment of organisms has hitherto been employed which comprises directly electrolyzing seawater to yield hypochlorous acid and effectively utilizing the hypochlorous acid for preventing the attachment of organisms. However, the discharge of untreated hypochlorous acid poses problems concerning environmental conservation because not only hypochlorous acid itself but also the organochlorine compounds and chlorine gas which generate upon decomposition of the acid are harmful. Consequently, use of hypochlorous acid is being increasingly restricted.
On the other hand, it has been reported that addition of a minute amount of hydrogen peroxide to the cooling water for use in power plants or factories is sufficiently effective in preventing the attachment of organisms. In addition, hydrogen peroxide decomposes only into water and oxygen, which both are harmless, to pose no problem to environmental hygiene.
However, hydrogen peroxide is unstable and incapable of long-term storage. Because of this and from the standpoints of safety in transportation and pollution abatement, there is a growing desire for an on-site hydrogen peroxide production apparatus. An electrolytic method has been proposed as a technique for on-site production of hydrogen peroxide.
In the electrolytic method, electrical energy, which is clean, can be used to cause a desired electrochemical reaction. By controlling the chemical reaction on a cathode surface, hydrogen peroxide can be produced. This electrolytically produced hydrogen peroxide has hitherto been widely used to decompose pollutants to thereby treat water for use in a particular application or to treat wastewaters. The electrolytic method enables the on-site production of hydrogen peroxide and eliminates the drawback in that hydrogen peroxide cannot be stored for long periods of time without a stabilizer. In addition, there is no need to take measures against the danger of transportation and pollution.
In the electrolysis of water in which oxygen is present, the reduction reaction of oxygen proceeds preferentially to yield hydrogen peroxide. When an electrolytic liquid itself is to be cleaned or sterilized, the electrolytic liquid comes into direct contact with an electrode to enhance the cleaning effect. There also are cases where superoxide anions (O2xe2x88x92), which are a highly active product of the reduction of one electron, are generated to improve the cleaning effect.
With respect to the electrolytic production of hydrogen peroxide, Journal of Applied Electrochemistry, Vol. 25, pp. 613-(1995) compares various processes for electrolytically yielding hydrogen peroxide. In each of these processes, hydrogen peroxide is efficiently obtained in an atmosphere of an aqueous alkali solution. It is therefore indispensable to use an aqueous solution of an alkali such as KOH or NaOH because of the necessity of supplying an alkali ingredient as a feed material. Formaldehyde decomposition as an example of the decomposition of organic substances with hydrogen peroxide is described in Journal of Electrochemical Society, Vol. 140, pp. 1632-(1993). Furthermore, a technique in which pure water as a raw material is electrolyzed using an ion-exchange membrane to synthesize ozone and hydrogen peroxide on the anode and cathode, respectively, is proposed in Journal of Electrochemical Society, Vol. 141, pp. 1174-(1994). However, these techniques are impractical because the current efficiency is low. Although a technique in which a similar method is conducted at high pressure to thereby heighten efficiency has been proposed, this technique is also impractical from the standpoint of stability. Moreover, an electrolytic method using a palladium foil has been proposed. However, this method is only useful in limited applications because the hydrogen peroxide concentration obtained is low and the method is costly.
In the treatment of tap water, well water, seawater, or other water containing multivalent metal ions in a large amount, there are cases where a hydroxide deposits on the cathode surface to give rise to problems such as, e.g., the inhibition of power feeding. For avoiding such problems, it is necessary to treat the water, e.g., tap water, to be supplied to an electrolytic cell with electrodialysis or a reverse osmosis membrane to diminish the multivalent metal ions, or to periodically clean the electrolytic cell main body with, e.g., an acid to remove the deposit. The levels of multivalent metal ions are 1 to 10 ppm for tap water, 1 to 100 ppm for well (ground) water and 500 to 5,000 ppm for sea water, respectively.
When feed water having a low electrolyte concentration as in soft water is used for electrolytically producing hydrogen peroxide, the current density is low and this method is hence unsuitable for the production of a large amount of hydrogen peroxide. In addition, an increased load is imposed on the electrodes, resulting in a shortened electrode life.
It is therefore an object of the present invention to meet the desire for a practical electrolytic cell capable of producing hydrogen peroxide at high efficiency over long period of operation.
The above object of the invention is achieved by providing an electrolytic cell for hydrogen peroxide production which comprises an electrolytic cell main body having an anode and a cathode both disposed therein and in which electrolysis is conducted while supplying to the electrolytic cell main body an oxygen-containing gas and a feed water containing at least one salt dissolved therein in a low concentration to thereby produce hydrogen peroxide. The invention further provides a process for producing hydrogen peroxide which comprises: converting a starting water containing multivalent metal ions into a feed water which is a low-concentration salt solution containing univalent metal ions by removing the multivalent metal ions from the starting water; and conducting electrolysis in an electrolytic cell main body partitioned into an anode chamber and a cathode chamber with a diaphragm while supplying the feed water and an oxygen-containing gas to the cathode chamber to produce hydrogen peroxide.